1. Field of the Invention
This invention relates to digital burst mode communication systems in general and, more particularly, to the apparatus and method of detecting valid data signals in said communication systems.
2. Description of the Prior Art
Burst mode communication systems typically use "packets" of digital data to communicate among the users of the system or network. The "packets" are bursts of data that one user sends to all other users of the system, with each user analyzing the received packet to see if it is addressed to it. Several popular local area networks, or LANs, utilize packets to communicate among the users thereof.
There are two prevalent topologies to a local area network: star or ring, or a combination of the two. Typically in a local area network there are several levels or hierarchies: a high-speed "backbone" and a slower local network coupling to the backbone through "repeaters" or multiplexers. An example of such an arrangement is shown in FIG. 1. Here, part of a ring or star network 1 is shown with the high-speed backbone medium 2 (typically a coaxial cable) and repeaters 3 for communication between the backbone medium 2 and users 4. The repeaters 3 communicate to the users 4 in a star configuration using a "slow" speed medium 5 (typically a twisted wire pair for each direction of communication) and corresponding transmit/receive interface units 6. The users 4, usually a computer or PC, can communicate to other users 4 either through the backbone 2 or within the repeater 3. The example of FIG. 1 is based on a local area network for implementing the Institute of Electronic and Electrical Engineers (IEEE) 802.3 10 Base-5, 2, F, or T specifications or proposed specifications for Carrier Sense-Multiple Access/Collision Detection (CSMA/CD) local area network. The 802.3 standard is what most system designers must conform to for compatibility with other manufacturer's systems in implementing 802.3 networks.
One of the most critical components in the network 1 is the digital data (packet) receiver in the interface units 6 and in the repeaters 3. These receivers are subject to more external noise and cross-talk than receivers on the backbone 2 since the transmission medium 5, here twisted wire pairs, are not self-shielding like coax cable. Regardless of what medium the receiver is coupled to, it should only accept packets that have predetermined characteristics, for example: have a minimum amplitude, have a minimum number of transitions, have frequencies above a minimum and below a maximum frequency, etc. These requirements increase the reliability of the system by rejecting noise and receiving only valid data.